B. Anderton Opti 521 Fall ‘08, HW 8 Pg. 1 of 2
Last Saved on 11/10/2008 11:26:00 AM
OPTI521/421 HW8
Problem 2: Short Answer
•
o Properties of PMMA: from Yoder, page 96
Linear thermal expansion coefficient: 51
610 K
α
−
−
=⋅
Refractive index: 1.492
r
n
=
Thermal refractive index change: CdTdnr°×−= −/105.10/ 5
o Focal length change calculation:
f
fT
β
Δ
=⋅⋅Δ
Coefficient of thermal expansion: from
()
1
1r
r
dn
ndT
βα
=− −, we
have 14
1074.2 −−
×= K
β
.
Focal length change: we find
mmKmmf 274.0)20)(50)(1074.2( 4=•=Δ −
•
o
()
(
)
(
)
51 3
6 10 K 50 10 m 20 K 0.06mmLLT
α
−− −
Δ= Δ= ⋅ ⋅ =
•
o Defocus
λ
428214.006.0274.0 =
=
−
=
Δ
−
Δ=Δ mmmmmmLfz
o Rms spot radius
mm
f
zD
FzD EP
n0185.0
50
5.12*214.0
*353.0*353.0/353.0 ==
Δ
=Δ=
